The large area plasma source has become an important tool for making a semiconductor and treating a material. The methods currently used for generating plasma include the DC discharge method, the RF discharge (13.6 MHz) method, and the microwave discharge (2.45 GHz) method. The DC discharge and the RF discharge methods are generally grouped into the capacitor-couple and the inductor-couple. The microwave discharge method involves the non-magnetic field microwave discharge and the magnetic field ECR. The radio frequency discharge method is most widely used in the process of making a semiconductor.
There are several advantages in using the microwave to generate the plasma in view of the fact that the plasma produced by the microwave has a higher plasma density and a higher ionization ratio, and that the quantities of the activated molecule and chemical radical are much greater in the microwave plasma than in the radio frequency plasma, and further that the microwave plasma is generated without the use of an electrode, and still further that the potential of the microwave plasma sheath is relatively low so as to minimize the pollution problem. It must be noted here that the microwave plasma can play an important role in the material treatment in light of the microwave plasma which contains the activated molecule and chemical radical in quantity.
In spite of the advantages of the microwave plasma, the microwave plasma is not used in the industrial application as often as the radio frequency plasma. The reason is that it is technically difficult to produce a large volume microwave plasma for industrial application. Such a technical difficulty as described above is attributable to the short wavelength of the microwave as well as the limited capability of the microwave to penetrate the plasma. The process of producing a large volume plasma calls for the use of the waveguide tube or the resonant cavity, which has a dimension greater than the wavelength of the microwave and the penetration depth of the microwave. As a result, the production of the plasma is convaneed to the position at which the microwave is introduced into the vacuum cavity, without the formation of the large volume plasma. In fact, the treatment of wafer in the process of making a semiconductor is carried out by the large area plasma, not by the large volume plasma. For this reason, the primary objective of the present invention is to provide a generator capable of producing a large area planarized plasma.
Weissfloch, et al. disclose in the U.S. Pat. No. 3,814,983 an apparatus for plasma generation by using a electromagnetic energy in the microwave frequency range, having a source of microwave energy, a strapped-bar slow wave structure, conveying means for conveying microwave energy from the source to the slow wave structure, and a plasma container. The vacuum reaction cavity is formed of a quartz tube having a diameter of 19 mm. As a result, this design is not suitable for treating a large area chip. Because of the use of the in-progress wave reactor, the power can not be completely used to excite the plasma. It is necessary to connect the rear end of the slow wave structure with a matching load to absorb the residual microwave power. As a result, the utilization rate of the power is relatively low. In addition, energy is used to excite the plasma at the time when the in-progress wave reaction is under way and when the microwave is moving forward. Therefore, the microwave power diminishes as the microwave moves forward. In order to attain a uniform plasma, the vacuum reaction cavity and the slow wave structure must be kept at a constant inclination, which must be adjusted in accordance with the operational conditions.
Komachi and Kobayashi disclose in the Japanese Patent 62-99481 a microwave plasma machine for treating a large area chip. The microwave structure is formed of a platelike slow wave structure made of Teflon (polytetrafluoroethylene). A large metal vacuum cavity is provided at the top thereof with a microwave window formed of a large area quartz plate. The microwave power is introduced via the microwave window from the slow wave structure into the vacuum cavity so as to excite the plasma. The main drawback is the slow wave structure which is made of polytetrafluoroethylene and is limited in the transmission of the large power microwave and in the heat resistance. In light of the application of the in-progress wave reaction, the electric field density so brought about is not as great as the electric field density brought about by the resonant cavity.